The armament of the RGB42A Class is excessive, yes. I expect the ship to be cramped. But I also expect this not to be a problem. Standard river patrols last only a few days and while in harbor the crews will be in baracks.
Here is a conceptional sketch I put together. The design is a lengthened RGB37A vessel, with a 3m section added and superstructure shifted a bid to make room for a second mount forward.
I think it does not look less realistic than other designs in WesWorld.
The armor suit of the RGB42A Class is meant to provide armored boxes for magazins and engines, plus some splinter protection at vital spots. More armor would have made the design too heavy. I am aware of the danger of unprotected buoyancy.
Regarding the TD42 Class - I have to check as its been a while since I put that one together. Either I had a clever idea or it is an error in my SS file.... Most likely the latter and you should assume a standard 4x2 pattern.